1. Field of the Invention
The present invention relates to a system for vapor growth and/or deposition and a method using the same, particularly to such the system provided with a gas supply-exhaust line system for supplying material gases and the like to a growth chamber and venting unnecessary gases therefrom, and a vapor growth and/or deposition method using the system.
More specifically, the present invention concerns to a vapor growth and/or deposition system, in which a single dummy line is arranged to a plurality of process gas lines to reduce the number of valve switching by half upon forming accumulated layers made from materials of different compositions so that pressure fluctuation in a main line due to the valve switching may be suppressed, thereby providing a formation of a super thin film having a stable crystallinity and an excellent interfacial steepleness, besides providing a low-cost, simplified structure by reducing the number of expensive block valves, and a vapor growth method using such the system.
2. Prior Art
A conventional crystal growth system has a structure shown in FIG. 5 [refer to Journal of Crystal Growth, 93,353 (1988), Suematsu, et. al. ]. Here, reference numeral 1 denotes a main line for supplying material gases to a crystal growth chamber. Numeral 2 denotes a vent line for discharging gases unnecessary for the crystal growth. Numerals 4, 6, 8, 10, 12 and 14 are air valves for controlling flows of the gases to the main line 1. Numerals 5, 7, 9, 11, 13 and 15 are another air valves for controlling flows of the gases to the vent line 2. Numerals 20, 22, 23, 25, 26 and 28 are mass flow controllers for controlling gas flow rates to adjust them to 100, 100, 180, 180, 90 and 90 ccm, respectively. Numeral 62 denotes a gas line A as a process gas line for supplying a process gas A. Numeral 60 is a dummy line A for supplying a hydrogen gas with a flow rate equal to that in the gas line A. Numeral 65 is a gas line B which is a process gas line supplying a process gas B. Numeral 63 is a dummy line for supplying a hydrogen gas with a flow rate equal to that in the gas line B. Numeral 68 is a gas line C which is a process gas line supplying a process gas C. Numeral 66 is a dummy line C for supplying a hydrogen gas with a flow rate equal to that in the gas line C.
As shown in FIG. 5, the gas lines, A, B and C, and the dummy lines A, B and C are communicated with the the main line 1 and the vent line 2 via the valves 4 to 15, respectively.
Now will be described an operation of the conventional vapor phase crystal growth system having such the structure set forth above. When the process gas A is not required for the crystal growth, the valve 6 of the gas line A is closed, while the valve 7 is opened. As a result, the process gas A is fed to the vent line so as not to contribute to the crystal growth. Meanwhile, the valve 4 of the dummy line A running the gas of which flow rate is controlled to be equal to that in the gas line A is opened, while the valve 5 is closed. Therefore, the hydrogen gas which has the same value of a flow rate as the process gas A is fed to the main line 1. Now, if the process gas A is desired to be fed to the main line, the gas line A has to connected to the main line 1 and the gas line B has to connected to the vent line 2 at the same time. So, the valves 4 and 7 are closed and the valves 5 and 6 are opened, simultaneously.
In consequence, a total gas flow rate of the gas supplied to the main line 1 is kept constant for a reason why the flow rate of the gas supplied to the main and vent lines 1 and 2 should not be varied, thereby suppressing fluctuation of the gas pressure in the main line 1 induced by the fluctuation of the flow rate. A stable pressure of the gas in the main line 1 becomes extremely important when a flow rate of the supplied process gas is very small.
In a gas supplying system failing in appropriate adjustment of the flow rates of gases supplied from dummy lines, a difference in gas flow rate between a process gas line and a dummy line causes a difference in gas pressure between the process gas line and a main line, when a process gas line having a smaller flow rate than the dummy gas line is connected to the main line. This leads to backflow of the gas from the main line to the process gas line when the process gas line has a small flow rate. In particular, it is necessary to switch the gas flows every few seconds, when an extremely thin crystal such as a quantum well structure is produced. In this occasion, such the backflow of the gas causes a gap of the timings between the valve switching and the actually supplied gas flow. This finally results in fluctuation in every crystal composition on the crystal interface (Hereinafter, this condition will be expressed as an inferior steepleness of a crystal, because of a slow switching of the gas.). A dummy line having a flow rate equal to that in a process line is required to obtain a thin crystal film excellent in steepleness, as described above.
On the other hand, the backflow of a gas is attributed to a small flow rate of the gas. So, there has been adopted a method, in which a process gas line having a small flow rate is connected to a line for a hydrogen diluent, as shown in FIG. 6, in order to keep the flow rate of the gas at a predetermined level or more.
In FIG. 6, numeral 1 also denotes a main line supplying material gases to a crystal growth chamber, 2 a vent line exhausting the gases unnecessary for the crystal growth. Numerals 6, 10 and 14 are air valves for controlling the gas flow to the main line 1, while 7, 11 and 15 are another air valves for controlling the gas flow to the vent line 2. Numerals 21, 22, 25 and 28 are mass flow controllers for controlling the gas flows to 200, i, 100 and 200 ccm, respectively. Numeral 61 is a diluting line A feeding a hydrogen gas for diluting the process gas A in the line A. Numeral 65 denotes a gas line B supplying a process gas B and 68 is another gas line C feeding a process gas C.
Hereinafter will be described an operation of an another conventional vapor phase crystal growth system having the structure shown in FIG. 6. When the process gas A is not required for the crystal growth, the valve 6 of the gas line A62 is closed, while the valve 7 of the same is in an open state. The process gas A is, as a result, fed to the vent line so as not to contribute to the crystal growth. If a gas of a great flow rate such as arsine as a constituent gas is switched to be supplied to the main line 1, the gas line A of a very small flow rate is directly subjected to the influence of the pressure increase in the main line due to the switching of the arsine gas so that the gas running in the main line 1 backflows into the gas line A62. To prevent this backflow of the gas from the main line, it is necessary to increase a flow rate in the gas line A62. For this purpose, a hydrogen gas is fed to the gas line A for dilution in order to increase the flow rate of the process gas A, so that the backflow of the gas due to the pressure increase may be suppressed. If the gas A is required for the crystal growth, the gas line A is connected to the main line. Accordingly, the valve 7 is closed and the valve 6 is opened, simultaneously.
Although a slight pressure fluctuation in the main line caused by the valve switching occurs, this will not cause a backflow of the gas running in the main line to the gas line A. This, however, causes a flow rate fluctuation of the gas running in the main line. No dummy line is disposed because such the gas line requiring a diluting gas line has a small flow rate.
Recently, a block valve system is used as integrated valves. The block valve system is a compact integration of a plurality of valves positioned as close to the main valve as possible to largely reduce a dead volume in the main line in the block valve system, thereby suppressing a change of the gas compositions because of stagnation of the gas therein. An interfacial steepleness is attained by a rapid change of the gas compositions. For this reason, the main line is required to have a dead volume as small as possible so as to prevent the stagnation of the gas.
The system having a structure shown in FIG. 5 requires the lines led to the block valve system, of which number is two times that of the process gas lines, since each of the process line requires a dummy line. That is, when the gas line A is switched over to the gas line B, the valves 4 to 11, 8 valves in total, must be switched so that slight difference in controllability of the valves occurs and this results in pressure fluctuation in the main line.
Moreover, there is a demand to decrease the number of the lines in the block valve system as much as possible, since a unit price per line of the block valve system is expensive.
In the structure shown in FIG. 6, on the other hand, it is impossible to fabricate a super thin film requiring a high-precision because of development of pressure and gas flow rate fluctuation in the main valve, although the block valve system includes only a small number of lines. To obtain a stable controllability relating to a film thickness of a crystal, a dummy line for stabilizing a pressure in the main gas becomes necessary to each process line, in addition to a diluting line for adjustment of the flow rate. This results in a system having a dummy line structure same as shown in FIG. 5. Hence, there will arise the same problem that it is impossible to decrease the number of the lines in the block valve system.
In consideration of these point, this invention provides a gas supplying line made up of a process line and a balance line, which balance line supplies an inactive gas such as a hydrogen gas necessary for giving a constant product of the viscosity and the flow rate to the gas supplied to the process gas line. Also such the balance lines equipped to the respective process gas lines contribute to retain the gases, running in a plurality of gas supplying lines not supplied to the growth chamber, at a constant product of the viscosity and the flow rate. Only one dummy line is required for this gas supplying group. Consequently, it is unnecessary to dispose dummy lines of [(the number of gas supplying line in a group)-1], and the number of the block valves decreases by [(the number of gas supplying lines in a group)-1]. It is therefore an object of the present invention to provide a vapor growth system, which not only enables reduction of the number of required gas supplying lines led to expensive block valves to about a half, but also enables formation of a super thin film having considerably stable crystals excellent in interfacial steepleness by reducing the number of times of the valve operation to a half, thereby comprising gas supplying line system having a simplified valve structure.